Dual-mode relay servos

THIS PAPER describes a dual-mode servo which utilizes a relay or contactor for both the saturated mode and the linear mode. The dual-mode servo, which was first suggested by McDonald,1,2 operates as an on-off or bang-bang controller for large amplitude errors (saturation mode) and as a linear servo when the error approaches zero (linear mode). The saturation mode may be optimized for a minimum transient response to step function inputs by means of special nonlinear circuitry which is used to determine the optimum points for control reversal, normally referred to as a switching line. The linear mode is optimized by inserting linear networks of the lead-lag variety.